1. Field of the Invention
The invention relates to methods and apparatus for testing piezoelectric actuators, particularly but not exclusively for piezoelectric actuators in magnetic heads-arm assemblies.
2. Description of Related Art
Piezoelectric actuators offer the user several advantages over other motion techniques. These include sub-nanometer displacement steps and high frequencies, very low power consumption in static operation, high reliability, and long life. Piezoelectric actuators achieve these characteristics because their motion is based on molecular effects within solid-state crystals.
Piezoelectric actuators rely on the piezoelectric effect, which has two versions. The xe2x80x9cdirectxe2x80x9d piezoelectric effect is the appearance of an electrical potential across certain faces of a crystal when it is subjected to mechanical pressure or distortion. The xe2x80x9cinversexe2x80x9d piezoelectric effect, is just the opposite: the crystal mechanically distorts in response to the application of an electric potential across certain faces of the crystal. In practical application, the xe2x80x9cdirectxe2x80x9d and xe2x80x9cinversexe2x80x9d piezoelectric effects interact with each other to yield complex motion and complex potential in response to transient excitation.
Because of their advantages, piezoelectric actuators are coming into widespread use in various industrial applications. In one particular use, manufacturers of disk head-arm assemblies are achieving extremely fine displacement resolutions by including piezoelectric actuators in the head-arm assemblies. Examples of such assemblies are shown in the following documents, all incorporated by reference herein:
U.S. Pat. No. 4,188,645
U.S. Pat. No. 5,189,578
Guo et al., xe2x80x9cA High Bandwidth Piezoelectric Suspension for High Track Density Magnetic Data Storage Devices,xe2x80x9d IEEE Transactions on Magnetics, Volume 34, No.4, pp. 1907-1909 (July 1988)
Fan et al., xe2x80x9cMagnetic Recording Head Positioning at Very High Track Densities Using a Microactuator-Based Two-Stage Servo System,xe2x80x9d IEEE Trans. on Ind. Elect., Vol.42, No. 3, pp. 222-233 (June 1995)
Miu, xe2x80x9cSilicon Micro-actuators for Rigid Disk Drives,xe2x80x9d Data Storage, pp. 3340 (July/August 1995)
Takaishi, et al., xe2x80x9cMicro-actuator Control for Disk Drive,xe2x80x9d IEEE Trans. on Mag., Vol. 32, No. 3, pp. 1863-1866 (May 1996)
Koganezawa, et al., xe2x80x9cA Flexural Piggyback Milli-Actuator for Over 5 Gbit/in.2 Density Magnetic Recording,xe2x80x9d IEEE Trans. on Mag., Vol 32, No. 5, pp. 3908-3910 (September 1996)
Mori, et al., xe2x80x9cA Dual-Stage Magnetic Disk Drive Actuator Using a Piezoelectric Device for a High Track Density,xe2x80x9d IEEE Trans. on Mag., Vol. 27, No. 6, pp. 5289-5230 (November 1991).
It can be seen that piezoelectric actuators can be used in head-arm assemblies in a wide variety of different structural formats. These include, but not limited to, structures using a single actuator and structures using two or more actuators; structures taking advantage of the linear expansion and contraction of the piezoelectric actuator directly and those which make use of the piezoelectric element only as part of bi-part strips; structures in which energization of the actuator(s) causes the head-arm effectively to lengthen or contract in the radial direction of the disk, and those which cause the head to move in the lateral direction substantially transverse to the radial direction of the disk. Many other structural formats incorporating piezoelectric actuators will be apparent.
FIGS. 1 and 2 also show an example of a head-arm assembly incorporating a piezoelectric actuator. FIG. 1 illustrates a Head Stack Array (HSA) 100 which includes five head-arm assemblies 110. FIG. 2 is a close-up view of one of the head-arm assemblies 110. It includes a flexure 112 extending outward from the end of an arm 114 of the HSA, and attached thereto by a beam 116. The beam flexes horizontally, thereby allowing the head 118 at the end of the flexure 112 to move slightly in either the left or right direction as indicated by the arrow 120. The left and right sides of the arm 112 are separated from the end of the arm 114 by respective piezoelectric crystals 122 and 124. The piezoelectric crystals 122 and 124 have electric field plates formed on opposing faces thereof. A control lead 126 is connected to one plate of crystal 122, and a second control lead 128 is connected to one plate of crystal 124. The second plates of crystals 122 and 124 are connected together and to a common third control lead 130. The crystals are driven differentially by a voltage source 132 in the body of the disk drive. It can be seen that as the voltage source goes positive, one of the piezoelectric crystals 122 and 124 will lengthen slightly and the other will shrink slightly, thereby causing a slight horizontal displacement of the head at the end of the head-arm 112. If the voltage source 132 goes negative, then the head will move slightly in the opposite horizontal direction. In one embodiment, the piezoelectric crystals on all the head-arms 110 on HSA 100 are connected together in parallel, such that all of the head-arms 110 move to the left or right in common. In other embodiment, each of the head-arm assemblies are operated independently from the others. Many other mechanisms for incorporating piezoelectric actuators into head-arm assemblies and HSAs are possible.
Disk drive component manufacturers typically test the various components of the disk drive before they are incorporated into assemblies or sub-assemblies. The piezoelectric crystals are among the many aspects of a head-arm assembly or HSA that must be tested. Several methods are known for testing piezoelectric actuators, though not specifically for magnetic disk head-arm assemblies. For example, Livingston U.S. Pat. No. 5,301,558, incorporated herein by reference, describes one such technique in which the piezoelectric crystal is placed in a testing apparatus which applies axial forced to the actuator. A fiber optics sensor determines the axial displacement of the actuator. A computer then uses this information to determine the effective modulus of the actuator. Aside from not being able to test parameters of an actuator desired for application in magnetic disk head-arm assemblies, the basic approach of this patent has an important disadvantage: the testing device tests the actuator itself, and not the assembly containing the actuator. This means that the actuator must be handled after testing and before being incorporated into the assembly, and this handling itself poses a risk of damage. It also means that the test cannot be used to detect damage or errors in the way that the actuator is eventually incorporated into the assembly. On the other hand, is often extremely difficult to develop tests of piezoelectric actuators which have been incorporated into assemblies, because the possibility for mechanical or even optical contact is very limited.
Lynas U.S. Pat. No. 3,786,348, incorporated by reference herein, describes a non-contact method for remotely indicating whether a piezoelectric transducer is properly connected in a circuit. Roughly described, and as understood, the method involves applying voltage impulses across the transducer and detecting the voltages produced by the transducer in response thereto. Impulse methods are disadvantageous, however, because they typically cannot apply sufficient energy to produce a meaningful result. Excitation energy levels can be increased by increasing the amplitude of the impulses, but such large amplitudes can sometimes risk destroying the piezoelectric device under test.
Other known methods for nondestructive testing of materials might also be usable for testing piezoelectric actuators in situ. Such known methods include continuous sine wave excitation within some frequency range, for example, but this technique typically requires a very large testing time because the tester must typically sweep the testing frequency through some range, performing tests at each of a number of discrete frequencies within that range.
Accordingly, there is a need for a method and apparatus for testing piezoelectric actuators in situ, which does not require physical or optical contact and with the actuators, does not involve high voltage impulse excitation, and does not require the lengthy testing time periods involved with sine wave excitation methods.
According to the invention, roughly described, piezoelectric actuators are tested as part of an assembly using a three-stage process. In the first stage, a steady-state electromechanical potential is induced into a piezoelectric member of an assembly. Typically this can be accomplished by applying a DC voltage across the crystal for long enough period of time for it to achieve (among other effects) a steady-state, or substantially steady-state, mechanical distortion. In the second stage, the electromechanical potential of the piezoelectric actuator is discharged rapidly but incompletely. The third stage begins with the abrupt termination of the rapid-discharge stage, thereby causing the crystal, and the voltage produced across it, to oscillate and decay freely. The voltage across the crystal continues to decay slowly in the third stage, and the oscillations continue to decay in magnitude as well, providing a relatively complex signal from which features can be extracted and compared to those of known-good devices. In an aspect of the invention, calibration of the testing system is avoided by extracting both time and frequency domain parameters from the Stage 3 output signal and comparing these to corresponding parameters of known-good devices.